To decide whether thin ascending limbs of Henle's loops participate in the process of hypertonic urine formation by active NaCl transport or by passive NaCl diffusion only, we shall develop two systems of partial differential equations of the renal medulla and all its structures. The two models will differ only in their representation of thin ascending limb function, and will be used to predict the transient response to different patterns of urea infusion to hydropenic animals, and to administration of antidiuretic hormone given to animals undergoing a water diuresis. Micropuncture experiments, based on the designs generated by the models, will be performed to decide which hypothesis is correct. To explain the behavior of the inner medulla in water diuresis, a hypothesis that vasa recta blood flow increases in water diuresis will be tested with the dual slit method of estimating erythrocyte velocity. Micropuncture experiments will be performed to test the hypothesis that thin ascending limbs reabsorb less NaCl in water diuresis. To test the hypothesis that interstitial pressure rises in states accompanied by increased rates of water reabsorption from collecting ducts, we propose to perfuse vasa recta in order to implement a null point detection method for estimating net interstitial forces acting on capillary fluid exchange. The method will be developed in peritubular capillaries of the renal cortex, and then applied to the vasa recta.